608 A. J. JUKES-BROWNE ON THE RELATIVE AGES OE 



lines of drainage proceeding from the chalk escarpment. Mr. S. Y. 

 Wood has recently accounted for these in a very different way, and 

 imputes to me the extraordinary postulate that I suppose a river to 

 be capable of converting the centre of its bed into a hill *. My 

 sections are simply illustrative of the fact that the general surface of 

 the country has been so lowered by the action of rain and rivers 

 since the time when the drainage was diverted from these old 

 channels, that the gravels of the older rivers now form ridges with 

 slopes on both sides ; but if one of these ridges is traced towards the 

 hill country, it gradually comes to occupy the position of a terrace 

 lying on one slope of a valley side. 



In the case of these old Devon gravels, the Witham on the one 

 side and the Brant on the other have cut down to lower levels, and 

 left the gravels of the ancient Devon on the top of the intervening 

 ridge. 



Strong confirmatory evidence, almost amounting to proof, of the 

 hypothesis that the Trent once flowed through the Lincoln gap, is 

 furnished by the composition of the gravels which occur on the 

 western side of the strip of .Fenland that forms the modern continua- 

 tion of the valley. These gravels will be described in the Geological 

 Survey Memoir on Sheet 83: but the point of interest in connexion 

 with my present subject is that they are largely made up of rounded 

 pebbles of quartzite, hornstone, and other old rocks, which have 

 evidently been derived from the Triassic pebble-beds of the west. 



The presence of these pebbles in such large quantities on the 

 eastern side of the Oolitic escarpment, and in gravels which border 

 the Witham valley, seems inexplicable except on the hypothesis of 

 their having been brought by the Trent through the gap at Lincoln. 



There is, therefore, every reason to believe that in early Post- 

 glacial times the Trent flowed along the course above indicated from 

 Newark to Lincoln, and from Lincoln south-eastward to the Fenland, 

 which was then probably an open bay. It remains then to indicate 

 the causes which seem to have operated in diverting the river from 

 the ancient course to its present channel. 



Diversion of the Trent. — The Humber flows in a transverse valley, 

 which has been cut down to a lower base-line than the Witham 

 valley at Lincoln ; and the rivers which now into it have always been 

 able to keep the passage open. Consequently the longitudinal valley 

 formed along the tract of soft Keuper marls has continually ex- 

 tended itself southward, aud the river in this valley must also have 

 shifted its channel continually eastward, as the escarpment of the 

 Bbsetics receded, and the Humber valley was cut down to lower and 

 lower levels. 



The river running northward along this valley would be supplied 

 by the brooks flowing from the west and draining the tract of 

 Bunter saudstones, and has ultimately developed into the river 

 Idle, which now rises near Mansfield, and pursues a north-easterly 

 course parallel to that of the Trent. 



I assume, therefore, that when the Trent ran via the Lincoln 

 * Quart. Journ. Geol. Soc. vol. xxxviii. p. 673. 



